1. Field of the Invention
The present invention relates to a plastic optical fiber and a process for the production of the same. More specifically, the present invention relates to a plastic optical fiber which can be used as optical fiber codes and optical fiber cables.
2. Description of the Related Art
Inorganic glass optical fibers have been known as optical fibers which are excellent in light transmission properties over a broad range of wavelengths. However, since said glass optical fibers are not good in processability or flexural strength, plastic optical fibers were developed and have been widely used as optical fibers.
These plastic optical fibers are comprised of a core polymer comprising a polymer which has excellent light transmission properties and a high refractive index, such as polymethylmethacrylate (hereinafter referred to as PMMA), polycarbonate (hereinafter referred to as PC), and a clad polymer comprising a transparent polymer which has a refractive index lower than that of the core polymer such as fluorine-containing polymer.
Known examples of the plastic optical fibers of this type are optical fiber strands, bulk fibers made by covering optical fiber strands with a functional protective layer, optical fiber codes made by covering optical fiber strands with a jacket, bundle fibers made of an assembly of bulk fibers and optical fiber cables made by applying tension members to the bulk fibers.
These plastic optical fibers, however, have many C--H bonds in the core polymer, and light absorption based on the expansion and contraction, or vibration of the C--H bonds appears at the regions of short wavelengths. Five to eight times the harmonic absorption also appears at the near infrared to visible ray region, namely, at a wavelength of not less than 400 nm. These serious light transmission losses in these regions have to be eliminated to use these conventional plastic optical fibers for light transmission. For example, the transmission loss of an optical fiber having a core of PMMA is about 100 dB/Km at a wavelength of 650 nm, and about 400 dB/Km at a wavelength of 780 nm. To avoid the transmission losses based on the C--H bonds in the core polymer, a core polymer comprising d.sub.8 -PMMA, of which all the H atoms in PMMA are replaced by D atoms, was proposed. This optical fiber containing d.sub.8 -PMMA as a core polymer has a transmission loss of 50 dB/Km at a wavelength of 780 nm. Deutrated PMMA, however, has high water absorbing properties, and over time the d.sub.8 -PMMA core polymer absorbs water, and the transmission loss will increase over time. An optical fiber showing such an increase in transmission loss cannot be used as an optical fiber, as an optical fiber is expected to have a high reliability over a long period.
At present LEDs that can emit rays in the near infrared region, and which have high power, and which can be used for high-speed data transmission have been produced in large quantities with a low cost. Since conventional plastic optical fibers, however, cannot use these LEDs as a light source for optical communications, light transmission beyond a wave guide length longer than 100 m cannot be accomplished with one optical fiber. Thus, LAN systems (Local-Area Network Systems) using plastic optical fibers, have not been so wide spread.
Recently, plastic optical fibers that can transmit rays in the near infrared region have been developed. For example, an optical fiber comprising a core polymer of a fluoroalkyl-.alpha.-fluoroacrylate polymer, and a clad copolymer of vinyliden fluoride and tetrafluoroethylene, was disclosed in EP 340557 A2 and EP 340555 A2. This optical fiber can transmit rays having a wavelength in the near infrared region, but its performance as an optical fiber is not satisfactory, since the difference in the refractive index between the core polymer and the clad polymer is not large enough to make an optical fiber having a large numerical aperture, and thus this optical fiber is not satisfactory as an optical fiber for transmitting data in a great amount. Further, because of its small numerical aperture, it is difficult for this optical fiber to inhibit the leakage of rays from its side surface when it is bent, and thus it is not satisfactory as an optical fiber for data communication.
Furthermore, the vinylidene fluoride-tetrafluoroethylene copolymer is not a perfect amorphous polymer, and has light-absorbing properties or light-scattering properties. Thus, an optical fiber containing this clad copolymer is not necessarily satisfactory in light transmission properties.